Conventionally, a method for producing a typical porous sintered article of titanium or titanium alloy is known which includes mixing a titanium or titanium alloy powder with an organic binder to obtain a mixture, molding the mixture to obtain a shaped article, heating the shaped article to remove the organic binder to obtain a degreased article (hereafter, this step in which the shaped article is heated to remove the organic binder to obtain a degreased body is referred to as the degreasing step), and further heating the degreased article obtained in the degreasing step at a high temperature, thereby obtaining a sintered article of titanium or titanium alloy.
Since it is impossible to perform a complete degreasing in the above-mentioned degreasing step, a very small amount of the organic binder remains in the degreased article which is obtained by degreasing the shaped article. It is known that, when this degreased article having a very small amount of the organic binder remaining is heated at a high temperature to obtain a sintered article of titanium or titanium alloy, some of the carbon atoms of the hydrocarbon react with titanium to form a carbide, and as a result, the obtained sintered article of titanium or titanium alloy has a structure in which titanium carbide compound having an average particle diameter of 1 μm or more is dispersed in the microstructure thereof, and the composition of the sintered article contains 0.2 to 1.0% by mass of carbon (see Japanese Unexamined Patent Application, First Publication No. 2001-49304). Although this sintered article of titanium or titanium alloy is generally porous, the porosity thereof is as small as 1% or less. Such a sintered article of titanium or titanium alloy having a small porosity can be used for various mechanical parts, but cannot be used as raw materials for various materials requiring high porosity, such as various filters, electrodes for fuel cells, and biomaterials.
In general, a raw material for various materials requiring high porosity, such as various filters, electrodes for fuel cells, and biomaterials needs to have a porosity of 50% or more. As an example of a method for producing a spongy sintered article having high porosity, the following method is known. To a metal powder are added and mixed an organic binder, a foaming agent and optionally a surfactant or the like to obtain a foaming slurry. Then, the obtained foaming slurry is molded into a shaped article, and the shaped article is dried by heating to foam the shaped article, thereby obtaining a green body having a porosity as high as 60% or more. Finally, the obtained green body having a high porosity is further heated at a high temperature to obtain a spongy sintered metal article having a high porosity. This spongy sintered metal article is known to have pores which open to the surface and continue with internal pores (hereafter, these pores are referred to as “continuous pores”), and a porosity of 50 to 98 volume % (see Japanese Unexamined Patent Application, First Publication No. 2004-43976 (“JP '976”).